Download Physical Geology Exam 4 Study Guide

Physical Geology Exam 4 Study Guide
Inclusion is a fragment of one rock type enclosed within another rock type.
Relative dating: The included rock must be older than the surrounding rock.
Group – A lithostratigraphic unit containing two or more Formations.
Formation – A body of rock strata that consists dominantly of a certain lithologic type, or combination
of types. It is the fundamental lithostratigraphic unit.
Member – A unit consisting of a specially developed part of a Formation.
Unconformities - Layers of rock deposited sequentially without interruption are conformable.
A break or gap in the geologic record, such as an interruption in the normal sequence of deposition of
sedimentary rocks. During this period, erosion removes some of the rock record.
3 TYPES;
Angular Unconformity - An unconformity in which younger sediments rest upon the eroded
surface of tilted or folded older rocks.
Disconformity - An unconformity between beds that are parallel. This term is usually applied to
erosional breaks that are represented elsewhere by rock units of at least one Formational rank.
Nonconformity - An unconformity between stratified rocks above and unstratified igneous or
metamorphic rocks below.
Correlation - demonstration of the equivalence of two or more geologic phenomena in different areas
(it may be lithologic or chronologic).
Correlation over short distances – locate the unit of interest in relation to other units above and below;
identify characteristic features.
Correlation over long distances is usually accomplished by using fossils.
FOSSIL DEFINITIONS
Petrification – organic matter is converted to rock by the infiltration of water containing dissolved
inorganic matter (CaCO3, SiO2) which replaces the original organic material, sometimes retaining the
original structure.
Permineralization – the original hard parts of an animal have additional mineral material deposited in
their pore spaces.
Replacement – the substitution of inorganic material for the original
organic components of a plant or animal.
Mold – an impression made in the surrounding earth or rock by the
exterior or interior of a fossil shell or other organic structure.
Cast – Secondary rock or mineral matter that fills a natural mold, thus
producing a replica of a fossil shell or skeleton.
Carbonization – The slow decay under water of organic material,
resulting in a concentration of carbon as a film showing the form and
structure of the original tissue.
Impression – If the film of carbon is lost from a fossil preserved in
fine-grained sediment, an impression (replica of the surface) may be evident.
Trace Fossils - A sedimentary structure consisting of a fossilized track,
trail, burrow, or tube resulting from the life activities of an animal.
(These are dinosaur footprints from Late Triassic –
Early Jurassic redbeds in Connecticut).
Burrows – tubes in sediment or rock left behind by a burrowing animal.
Gastroliths – polished “stomach stones” from extinct reptiles.
Coprolite – fossilized animal feces.
FOSSIL PRESERVATION
Certain environmental conditions favor fossil preservation.
1. Rapid burial – prevents organism from being either eaten by scavengers or decomposed by bacteria.
2. Hard parts – hard parts of an organism (shells, bones, teeth) are more likely to become fossilized.
In the late 1700’s and early 1800’s English engineer William Smith (1769-1839) observed
that different layers of rock contained different fossils. He first worked at Mearn’s Pit (coal),
then worked as a surveyor for a canal company. His travels and interests led to him develop
a large fossil collection, and the nickname “Strata Smith”.
Principle of Fossil Succession: Fossil organisms succeed one another in a definite and determinable
order. Any time period can be recognized by its fossil content.
Index Fossil: A fossil taxon (usually a Genus, rarely a Species) that identifies and dates the strata in
which it is found. Typically, an index fossil is limited to a short span of geologic time.
Radiometric dating – Calculating the age in years for geologic materials by measuring:
- the presence of a short-life radioactive element such as carbon-14;
- a long-life radioactive element plus its decay product, such as potassium-40/argon-40.
Radiometric dating allows geologists to evaluate the dates of formation of very old materials.
Radioactivity -The emission of energetic particles and/or radiation during radioactive decay.
Radioactive decay – The spontaneous disintegration of the atoms of certain nuclides into new nuclides,
which may be stable or undergo further decay until a stable nuclide is finally created.
Nuclide – A species of atom characterized by the number of neutrons and protons in its nucleus.
The initial isotope, prior to radioactive decay, is called the parent element. The isotope that forms
following radioactive decay is called the daughter element.
Examples:
Parent – Uranium 238 is the parent element of lead 206
Daughter – radon is the daughter element of radium
Alpha Emission – alpha particles (2 protons, 2 neutrons) are emitted from the nucleus.
Beta Emission – A beta particle is an electron. Therefore, emission of a beta particle results in no mass
change, but the atomic number increases by one (the electron comes from a neutron, thus changing one
neutron to a proton).
Electron capture – If an electron is captured by a nucleus it will combine with a proton to form a
neutron, resulting in no mass change, but the atomic number decreases by one as one proton is
converted to a neutron.
Half Life
The time required for half of the nuclei in a sample to decay.
When parent-to-daughter is 1:1, then one half-life has passed.
When parent-to-daughter is 1:3, then two half-lives have passed.
When parent-to-daughter is 1:7, then three half-lives have passed.
Pb 212 – Pb 208
Rb-Sr Dating
Rubidium has 32 isotopes (naturally occurring Rb85 (72.2%) and radioactive Rb87 (27.8%).
Different minerals in a given geologic setting can have a distinctly different ratio of strontium-87 to
strontium-86 (87Sr/86Sr) as a consequence of different ages, original Rb/Sr values and the initial
87
Sr/86Sr.
If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement
of the Rb and Sr concentrations and the Sr87/Sr86 ratio.
The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.
Carbon 14 Dating
Stable isotopes: C12, C13
Unstable C14 is produced in the upper atmosphere by cosmic ray fluxes.
Cosmic rays bombard gas atoms, releasing neutrons which are absorbed by nitrogen atoms (N=7, mass =
14), causing each nucleus to emit a proton (N=6, mass = 14) and create C14.
The C14 circulates and is incorporated into living matter.
While alive, organisms maintain constant C14 to C12 proportions. Upon death, C14 decreases by decay
to N14 (beta emission). Comparison of C14 to C12 allows estimation of age.
C14 dating is good for wood, charcoal, bones, f lesh, cotton fabrics.
MASS WASTING
Gros Ventre Landslide (Kelly, Wyoming)
On June 23, 1925, one of the largest fast-moving landslides in generations occurred
near the village of Kelly, Wyoming. In just three minutes, huge amounts of rock and
debris cascaded down the north slope of Sheep Mountain, changing the area forever.
Hurling down the slope at 50 m.p.h., the mile-wide slide carried 50,000,000 cubic yards
of debris. The mass rode 300 feet up the opposite slope, blocked the Gros Ventre River,
and formed a five-mile long body of water known today as Lower Slide Lake. The piles
of debris seen today contain large chunks of Tensleep Sandstone, along with remnants
of the original forest.
CAUSES;
-
Heavy rains and rapidly melting snow saturated the Tensleep Sandstone, causing the Amsden
Shale rock layer on Sheep Mountain to become exceptionally slippery;
The river, cutting through the sandstone, produced a “free side” with no extra support holding it
in course;
Swampy pools with no outlets, on top of the mountain, indicating water-saturated soil.
Earthquake tremors (which were occurring) added to these already unstable factors and could
have precipitated a landslide.
Mass Wasting - The downslope movement of soil, regolith, and rock material under the
influence of gravity.The most spectacular examples of mass wasting occur in newly formed, rugged
mountains. These regions are rapidly eroded by rivers and glaciers into regions of steep and
unstable slopes.
Factors:
Gravity, Saturation of soil or rock materials, Slope oversteepening, Removal of vegetation, Ground
vibrations (earth quakes)
Saturation – The degree to which the pores in a rock contain oil, gas, or water, generally
expressed in percent of total pore space.
Angle of repose – The maximum slope or angle at which loose, cohesionless material remains
stable (typically between 33 and 37 degrees). Larger, more angular fragments maintain the
steepest slopes.
Activities leading to oversteepened slopes:
-a stream undercutting a valley wall
-wave action on a cliff face
-manmade slopes (retaining walls, etc.)
Effects of Vegitation
-
Raindrops hitting the soil surface can seal the soil particles and make a crust that prevents
infiltration and creates runoff.
Trees and shrubs intercept precipitation before it hits the soil surface.
Most of the intercepted precipitation evaporates back into the atmosphere, and the moisture
that drips off the plants causes little soil damage because it has less force.
Roots physically reinforce soils, resist erosion, and increase infiltration of water into the soil.
Roots form physical pathways that help water infiltrate the soil.
Vegetation actively decompacts soil through the expansion of the root systems and the addition
of organic matter to the site. Water absorbs more readily into uncompacted soil.
Earthquakes
An earthquake and subsequent aftershocks may often serve as a trigger for a mass wasting event.
Soil liquefaction is a phenomenon whereby a saturated soil substantially loses strength and stiffness in
response to an applied stress, and behaves like a liquid. Earthquake shaking or other sudden changes in
stress conditions cause soil liquefaction.
Landslides
Landslides may occur without obvious trigger events.
The long-term effects of weathering, moisture retention and other physical processes reduce slope
stability to the point where failure occurs in an apparently random fashion.
Old Man of the Mountain, New Hampshire
For decades, the Old Man was the official symbol of the state of New Hampshire.
After thousands of years in the making, the rock formation was "discovered" by white settlers at the
beginning of the 19th century. The profile was formed by several ledges of granite, and in a cubist
manner the representation is striking. The face was only about 40 feet high. It was set
on a cliff 1,000 feet above the valley floor.
Freezing and thawing opened fissures in the Old Man's forehead.
By the 1920s, the crack was wide enough to be mended with chains, and
in 1957 the state legislature passed a $25,000 appropriation for a more
elaborate weatherproofing, using 20 tons of fast-drying cement, plastic
covering, and steel rods and turnbuckles, plus a concrete gutter to divert
runoff from above. A team from the state highway and park divisions kept
the patchwork up to date each summer.
Nevertheless, the formation collapsed between midnight and 2 a.m.,
May 3, 2003. The Old Man of the Mountain is no more.
Mass Wasting Processes
Types of mass wasting are classified based on the material involved, and the kind and velocity of
movement.
Material – soil, regolith, rock
Motion – fall [talus slopes], slump/slide[rotational and translational], flow [lobes or tongues of material]
Rate – fast [avalanche], slow [creep]
Slump
Movement of a mass of material along a curved, concave
rupture surface.Slump is considered a rotational slide.
Causes:
-oversteepened slopes.
-overloaded slopes
(weaker clays beneath
sandstone for example).
Talus slopes develop in rock fall zones.
Rock/Debris Slide
Rock and debris slides are translational slides.
A moving mass of rock fragments, soil, and
mud, with more than half of the particles being
larger than sand size.
Speed: slow [1 m/year]
fast [160 km/hr = 100 mph]
Debris flows tend to occur frequently in
semiarid mountainous regions.
Lahar (fast) A mudflow of pyroclastic material
on the flank of a volcano.
Creep (slow) The gradual downhill movement of
soil and regolith.
Solifluction (very slow) The slow (0.5 to 5.0 cm/yr)
downslope movement of waterlogged soil. Solifluction is
common in areas where water cannot infiltrate
soils, and in areas of permafrost.
Permafrost permanently frozen soil or subsoil.
Permafrost underlies about 1/5 of the Earth’s land area!
Red River flooding, MN/ND
During the spring of 2001, major flooding occurred for the second time in four
years on the Red River of the North and its many tributaries in eastern North
Dakota and western Minnesota.
Unlike the 1997 floods, which were the result of record-high snowpacks
region-wide and a late spring blizzard, the 2001 floods were the result of
above-average soil moistures in some areas of the basin, rapid melting of
above-average snowpacks in the upper basin, and heavy rainfall that swept
across the region on April 7, 2001.
In the fall of 2000, soil moistures varied across North Dakota
and Minnesota.
Before freeze-up in November 2000, a series of storms saturated the upper
portions of the soils preventing further infiltration of moisture and leaving
many parts of the Red River Basin with 1 to 2 feet of snow.
Along the Red River, the 2000-2001 snowfall was above average but was less
than the record 1997 snowfall.
Temperatures began to warm during the end of March and caused the flooding
to start in the upper Red River Basin.
A massive storm system that brought heavy rains and high winds moved through
the upper plains on April 6 and 7, 2001.
The upper part of the basin received 1 to 2 inches of rain that added to the flooding
problem. Rainfall continued periodically throughout April in parts of the Red River Basin.
Fargo/Moorhead
• On April 14, 2001, the peak stage of the Red River at Fargo, N. Dak., was 36.63 feet, which is 3.09 feet
less than the record set in 1997, and the peak discharge was 20,300 cubic feet per second.
Grand Forks/East Grand Forks
• High discharges continued downstream on the Red River. On April 14, 2001, the peak stage of the Red
River at Grand Forks, N. Dakota, was 44.87 feet, which is 9.48 feet less than the record set in 1997. The
peak discharge was 55,800 cubic feet per second, which occurred on April 11, 2001.
-----------------------------------------------------------------------------------------------------------------------------------------Hydrologic Cycle – The process of water evaporating from oceans, plants, and soils; condensing and
falling as precipitation; percolating through soil and eventually returning to the ocean.
Infiltration – The process by which water enters the ground.
Evaporation – The vaporization of a liquid occurring only at the surface of the liquid.
Transpiration - The loss of water vapor from parts of plants (leaves, stems, flowers, roots).
Evapotranspiration - The sum of evaporation and plant transpiration from the Earth's land surface to
atmosphere.
Stream – water that flows in a channel. Synonyms: branch, brook, beck, burn,
creek, "crick", gill, kill, lick, syke, bayou, rivulet, streamage, wash, run or runnel.
River – a stream that carries substantial amounts of water and has
numerous tributaries.
Drainage Basin -The tract of country that contributes water to a particular
stream channel or system of channels, or to a lake, reservoir or other body of water.
Watershed - The terms “watershed” and “drainage basin” are often used
interchangeably in North America. In Great Britain, however, “watershed
is used to denote a drainage divide.
Drainage Divide - A drainage divide is the boundary between drainage basins.
Stream Flow
Laminar flow – Water flow in which the stream lines remain distinct.
Turbulent flow – Water flow in which the flow lines are confused and mixed.
Flow Velocity
Factors influencing stream flow velocity:
- channel slope or gradient (v/h);
- channel size and cross-sectional shape;
- channel roughness; and
- the amount of water flowing in the channel.
Measuring Flow Velocity
Gaging stations equipped with current meters are used to measure flow velocity.
Calculating Flow Rates
A basic flow rate equation:
Q = VA
Q = flow rate (cubic meters per second, or m3/sec)
V = velocity (m/sec)
A = area (m2)
For a rectangular open channel:
Q = VBh
Q = flow rate (cubic meters per second, or m3/sec)
V = velocity (m/sec)
B = channel width (m)
h = depth of flow (m)
For Channel B
Q = VBh
Q = flow rate (cubic meters per second, or m3/sec)
V = velocity (m/sec) – let’s say 5 m/sec
B = channel width (m) – 5 m
h = depth of flow (m) - 2 m
Q = (5 m/sec)(5m)(2m) = 50 m3/sec
The flow rate is also known as discharge, or the volume of water
flowing past a point over a given unit of time.
Stream Profile - A cross sectional view of a stream from its source to its mouth.
Erosion Ability
When in a channel, the erosional power of flowing water is related to the stream slope and discharge.
The rate of erosion is dependent on the bank and bed materials.
In bedrock:
Quarrying- removal of blocks from the stream channel
Abrasion – sediment grains entrained in the flow abrade the channel. Example: potholes
Corrosion – a chemical weathering/dissolution process, typical of carbonate rocks.
Dissolved Load – That part of the total stream load that is carried in solution.
Suspended Load – The part of the total stream load that is carried for a considerable period of time in
suspension, free from contact with the stream bed. Typically consists of clay, silt, sand.
Bed Load – The part of the stream load that is moved on or immediately above the stream bed, such as
the larger or heavier particles (boulders, pebbles, gravel) rolling and saltating along the bottom.
Stream Channels
Bedrock Channels
Alluvial Channels
Alluvial Channels
Meandering Channels – wide bends, primarily suspended load transport of
silt and clay, sand, and fine gravel.
Associated terms:
Cut bank – outside bank of a meander, zone of erosion.
Point bar – inside bank of a meander, zone of deposition of coarser material.
Cutoff – short circuit of the channel through a narrow neck of land separating
two adjacent segments of the channel.
Oxbow lake – An abandoned meander bend.
Braided Channels – a complex network of converging and diverging channels.
They form when a stream load is primarily coarser sand and gravel.
Typical at the end of glaciers.
Oxbow Lake - A crescent-shaped body of standing water situated in an abandoned channel of a
meander.
Base Level - The theoretical limit or lowest level toward which erosion of the
Earth’s surface constantly progresses, but seldom, if ever, reaches. The ultimate
base levelis sea level.Local or temporary base levels include lakes, resistant layers
of rock, rivers, and dams.
Formation of Valleys
Valley deepening occurs when:
1. there is a steep stream gradient; and
2. the channel is well above base level.
The result is a V-shaped valley.
Valley widening begins to occur when:
1. down cutting decreases; and
2. the stream becomes graded. A stream is graded when
the stream channel characteristics are such that it is able to
maintain the minimum velocity to transport the material
supplied to it).
Continued lateral erosion results in:
1. a meandering channel; and
2. development of a floodplain.
Incised Meanders - Incised meanders typically
form in response to changes in base level.
Terraces - A relatively level bench or step-like surface
breaking the continuity of a slope. Fluvial terraces are
remnants of a former floodplain.
Deltas
A fan-shaped fluvial plain that forms at the mouth of a river. Deltaic sediments are deposited as three
types of beds:
Foreset - coarse particles that settle almost immediately when the flow rate
decreases. Foreset beds slope downcurrent from the delta front.
Topset – thin, horizontal, coarse sediments deposited
during flood stage.
Bottomset – finer sediments (silt, clay) that settle
away from the mouth of the river in nearly horizontal beds.
Alluvial Fans
Fan shaped deposits of coarse alluvium deposited by a stream
especially in arid or semi-arid regions where a stream flows
out of a narrow canyon.
Alluvium – detrital deposits made by streams on river
beds, flood plains and alluvial fans.
Drainage Patterns
Dendritic – streams branch randomly at almost any angle; like
tree branches. Cause: relatively uniform bedrock.
Radial – streams diverge from a central area. Typically develops
on volcanoes and areas of dome-like uplift.
Trellis – parallel main streams with right-angle tributaries.
Cause: forms along edges of alternating hard and soft rock
outcrops.
Rectangular – drainage pattern with many right angle bends.
Cause: highly jointed bedrock.
Water Gap - A deep pass in a mountain ridge, through
which a stream flows. The stream may be either an
antecedent, or superposed/superimposed stream.
Antecedent Stream
Superposed Stream